Desensitizer for exposed silver halide emulsions



United States Patent Office 3,314,790 DESENSITIZER FOR EXPOSED SILVER HALIDE EMULSIONS Herbert B. Cowden, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Sept. 23, 1963, Ser. No. 310,897 23 Claims. (Cl. 96-62) This invention concerns a desensitizer and a process of using it to render exposed silver halide emulsions desensitized to roomlight.

Photographic emulsions of projection speed cutomarily have to be developed in darkness or under safelight conditions, since roomlight handling of the exposed emulsion results in fogging the emulsion. It has been desirable to find a means of treating the silver halide emulsion after it had been exposed in order to. preserve the latent image but to desensitize the emulsion so that following this desenitizing treatment it could be processed in ordinary illumination such as roomlight. This has been particularly desirable in projection speed photographic elements used in the graphic arts wherein mechanical operations are required in preparing printing plates. Typical silver halide emulsions of this type for use in colloid transfer are described in Clark et al., U.S. Patent 2,763,553.

In the diffusion transfer system, as described, for instance, in Rott US. Patent 2,352,014, it has also been desirable to desensitize an exposed silver halide emulsion so that the emulsion can then be developed and contacted against a receiver sheet in roomlight.

Various desensitizers have been known in the past, but have presented problems which rendered them relatively unsatisfactory for general use. For instance, some desensitizers are not sutficiently potent to be effective unless relatively large quantities are used. Still others interfere with the rate of tanning development, so that they are not satisfactory for use in a colloid transfer system. Others interfere with the pH when used in an activator bath to process silver halide emulsions containing developer, or affect time and temperature latitudes or exhaustion life of activator solutions. Therefore, there has been a need for an inexpensive, water-soluble, potent desensitizer suitable for use with various photographic systems.

i l have found a class of compounds which can be used as a desensitizer in various photographic systems including diffusion transfer, colloid transfer, etc.

One object of this invention is to provide a desensitizer .which can be added to activator solutions for use in the colloid transfer system without adversely affecting the properties of the activator solution. A further object is to provide a desensitizer for exposed silver halide emulsions which permits roomlight handling without adversely affecting the latent image. An additional object is to provide a desensitizer for use in diffusion transfer systems Which desensitizes the exposed silver halide emulsion and permits roomlight developing of the silver halide emulsion.

In attaining the above objects, I use a 2,2'-azopyridine in a bath preferably containing about 0.10 to 050 gram per liter. 2,2-azopyridine compounds employed in my invention can be prepared by the methods of Kirpal Ber. 67B, 70-1 (1934) CA. 28:1701 and Bystritskaya, Kirsanon, J. Gen. Chem. (USSR), 10, 1101-1107, #12 (1940); CA. 351402-37 (1941). The exposed silver halide emulsion is immersed in an aqueous solution containing the desensitizer until the silver halide emulsion has been sufliciently desensitized to permit handling in roomlight, after which, it can be passed through the appropriate processing solutions and mechanical steps.

3,314,790 Patented Apr. 18, 1967 It will be appreciated that the length of time during which the silver halide emulsion is immersed in the desensitizing bath will depend on the silver halide emulsion, but the preferred time is from 12 to 20 seconds. The temperature of the bath is not critical and may be used at a temperature of from 60-85 F. depending upon convenience and the ambient room temperature.

2,2'-azopyridine compounds which are operative include all those which have a solubility of at least 010 gram per liter. These include 5-chloro-2,2'-azopyridine as Well as those which include solubilizing groups. The operation of the desensitizer does not depend upon the pH and it may be used in an aqueous bath alone or may be incorporated in other baths such as activator or developing baths. I i

The following examples are intendedto illustrate my invention but not to limit it in any way.

Example I A silver halide emulsion of the type described in US. Patent 2,763,553 was exposed by projection to a two'- tone subject and immersed in an activator having the following composition:

Z-diethylaminoethanol NaHCO g 7.5 Sequestrene Na4 g 3.4 Nigrosine black dye g 0.75 Water to make liters 1.0

Example 2 Example 1 was repeated using the same emulsion but with the addition to the activator of 0.25 gram of 2,2'- azopyridine. After immersion in the activator for 20 seconds, the silver halideemulsion could be exposed to as much as 50 foot-candles of fluorescent light for 20 to 30 seconds without fogging and attendant tanning of image areas. Transfers to lithographic plates using the modified activator solution were made and theresults Example 3 A silver chlorobromo emulsion coating was exposed to an image and immersed for 20 seconds in Bath 1 described below. The matrix was withdrawn from Bath 1 in roomlight and immersed from 5 to 10 seconds in Bath 2. After being withdrawn from Bath 2, the silver halide emulsion coating was rolled in contact with a nucleated receiving sheet. The two sheets were separated after about 20 seconds of contact to provide a positive image on the receiving sheet.

BATH 1 2-diethylaminoethanol ml 37.5 NaHCO g 7.5

Ethylenediamine tetraacetic acid, sodium salt grams 3.4 2,2'-azopyridine g 0.25 Nigrosine g- 0.75 Water to make ..liters 1.0

The positive image was of high quality and high density. After the image activation, the steps were carried out under 4050 foot-candles of fluorescent light and the results were identical to those obtained when the whole process was carried out under red safelights.

The desensitizers of the invention also can be used in conventional photographic developers to allow photographic coatings to be processed in roomlight or under bright safelights.

Example 4 A coarse-grained silver bromoiodide emulsion of the type normally used in medical X-ray films, was coated on a cellulose acetate film support. Samples of this coating were exposed on an Eastman 1b sensitometer, processed under the levels of safelight illumination identified below as Conditions A and B, and developed for 3 minutes using Kodak Rapid X-Ray Developer with and without the addition of 2,2'-azopyridine.

SAFELIGHT LEVEL Condition A.-25-Watt bulb and Kodak darkroom lamp containing a Wratten No. 2 filter with the lamp 3 feet from the film surface.

Condition B.25-watt bulb and Kodak darkroom lamp containing a Wratten OA filter with the lamp 1 foot from the film surface.

The following results were obtained:

developing-out emulsions and may have the binders known in the art including proteinaceous binders, synthetic resins, etc. Moreover, the desensitizers may be used in other embodiments than by immersing the silver halide emulsion in a bath. For instance, a spray or wick application may be used or similar application, where the desensitizer is applied to the surface of the emulsion. This may be particularly useful in rapid processing utilizations. The desensitizers may also be added to monobaths used in conventional silver halide processing as well as in those baths similar in nature which are used for diffusion transfer systems.

The support for the silver halide emulsions used in connection with this invention are not critical and include those known in the art such as paper, glass, metal, cellulose ester, synthetic resins, etc. Solubility in water of the desensitizer should be at least 0.10 gram per liter of water at room temperature or about'70" F.

The solubilizing groups Which may be used to solubilize the azopyridines of this invention include those which are known to be useful and compatible with photographic emulsions including carboxyl, sulfonic, halogen, etc.

The preferred desensitizers of this invention are those which do not interfere with the developability of the latent image of the emulsion. Moreover, they should not bleach out the latent image. When used with chemical transfer, they should not interfere with the transfer of the solubilized silver halide or with the toning. When used with colloid transfer, they should not interfere with the tanning or with the transfer of the soft colloid. In addition, they should not interfere with the density of the image.

It will be appreciated that the preferred desensitizer is 2,2-azopyridine but that the 2,2-azopyridines containing solubi-lizing groups are useful for desensitizing photographic emulsions depending upon the particular emulsion and the system in which it is being used, and that certain of these compounds may be more preferable in one system than in another.

TABLE Developer Developer Plus 100 mg. of

2,2-Azopyridine Safelight Condition Relative Gamma Fog Relative Gamma Fog Speed Speed Condition A 191 1. 0.17 174 1. 48 0. 18 Condition B 100 0. 63 0. 87 191 1. 28 0. 25

It will be appreciated that this desensitizer can be used with various silver halide emulsions including silver chloride, silver bromide, silver iodide, silver chlorobromide, silver chloroiodide and silver chlorobromoiodide. When this desensitizer is used in the diffusion transfer process, the paper receiving sheets may be used as well as other types of receiving supports including transparent supports such as polyethylene, polystyrene, and the like, provided these supports have been nucleated with a silver precipitating agent of the type known in the art. Although Example 2 illustrates the use of the desensitizer with an unhardened silver halide emulsion, it is clear that hardened silver halide emulsions may also be used.

This desensitizer is of particular use in connection with X-ray processing, since it can be added to the developer bath and after a short period of time the processing can be continued using roomlight. This is particularly advantageous in dental X-ray work where it is often desirable to see the developed image by means of white light very shortly after the X-ray film has been exposed.

By roomlight, as used herein, is intended visible White light, including daylight, tungsten light, fluorescent light, etc.

The desensitizers of this invention may be used with silver halide emulsions which are known in the art as The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. An aqueous solution for desensitizing an exposed silver halide emulsion containing a silver halide developer and a desensitizing amount of a 2,2-azopyridine having a solubility in water of at least 0.10 gram per liter.

2. An aqueous solution for developing an exposed silver halide emulsion containing at least one silver halide developer capable of reducing exposed silver halide to silver and a desensitizing amount of a 2,2-azopyridine selected from the class consisting of 2,2-azopyridine and S-chloro- 2,2-azopyridine.

3. An aqueous solution of claim 2 containing 0.10- 0.50 gram per liter of the said 2,2-azopyridine.

4. An aqueous solution of claim 2 in which the said azopyridine is 2,2-azopyridine.

5. An aqueous solution of claim 2 in which the said azopyridine is 5-chloro-2,2-azopyridine.

6. An aqueous solution of claim 2 containing at least ble silver halide complex in which the said one silver halide solvent capable of forming a water soluble silver halide complex.

7. An aqueous solution of claim 2 containing at least one silver halide solvent capable of forming a Water soluble silver halide complex in which the said azopyridine is 2,2-azopyridine.

8. An aqueous solution of claim 2 containing at least one silver halide solvent capable of forming a water soluazopyridine is 5-chloro-2,2'-azopyridine.

9. An aqueous solution of claim 2 containing at least one silver halide solvent capable of forming a Water soluble silver halide complex and 010-050 gram per liter of the said 2,2-azopyridine.

10. An aqueous solution of claim 2 containing at least one silver halide solvent capable of forming a Water soluble silver halide complex and 010-050 gram per liter of 2,2-azopyr idine.

11. An aqueous solution of claim 2 containing at least one silver halide solvent capable of forming a Water soluble silver halide complex and 010-050 gram per liter of 5-chloro-2,2-azopyridine.

12. A process for desensitizing a silver halide emulsion comprising imbiding into the exposed silver halide emulsion an aqueous solution containing a desensitizing amount of a 2,2'-azopyridine selected from the class consisting of 2,2'-azopyridine and 5-c-hloro-2,2'-azopyridine having a solubility in water of at least 0.10 gram per liter, to render the silver halide emulsion desensitized to room-light.

13. A process of claim 12 in which dine is 2,2'-azopyridine.

14. A process of claim 12 in which the said azopyridine is 5-chloro-2,2-azopyridine.

15. A process of claim 12 in which the aqueous solution contains 0.10-050 gram per liter of the said 2,2- azopyridine.

16. A process of claim 12 in which the aqueous alkaline solution contains 0.10-0.50 gram per liter of 2,2-azopyridine.

1 7. A process of claim 12 in which the aqueous solution contains 0.10-0.50 gram per liter of 5-chloro-2,2'- azopyridine,

the said azopyri- -18. A process of claim 12 in which the aqueous solution contains a silver halide developer capable of reducing exposed silver halide to silver.

19. A process of claim 12 in which the aqueous solution contains at least one silver halide developer capable of reducing exposed silver halide to silver and a desensitizing amount of 2,2'-azopyridine.

20. A process of claim 12. in which the aqueous solution contains at least one silver halide developer capable of reducing exposed silver halide to silver and a desensitizing amount of 5-chloro-2,2-azopyridine.

21. A process of claim 12 in which the aqueous solution contains at least one silver halide developer capable of reducing exposed silver halide to silver and a silver halide solvent capable of forming a water soluble silver halide complex.

azopyridine.

References Cited by the Examiner UNITED STATES PATENTS 2,673,152 3/1954 Allen et al. 96-109 FOREIGN PATENTS 871,938 7/1961 Great Britain.

OTHER REFERENCES NORMAN G. TORCHIN, Primary Examiner. Q. E. DAVIS, Assistant Examiner, 

1. AN AQUEOUS SOLUTION FOR DESENSITIZING AN EXPOSED SILVER HALIDE EMULSION CONTAINING A SILVER HALIDE DEVELOPER AND A DESENSITIZING AMOUNT OF A 2,2''-AZOPYRIDINE HAVING A SOLUBILITY IN WATER OF AT LEAST 0.10 GRAM PER LITER. 